In this paper, the principle of VoIP( Voice over IP) and the architecture of IP phone system as well as its standard are discussed. The key technologies of VoIP and the methods to improve IP phone system are proposed. It shows that the VoIP not only can provide the telecommunication servicewith quality of service( QoS) such as telephone and fax, but also gives more and more increment value service such as on line and interactive information. It has bright market perspective.